The Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”) is a well known type of semiconductor transistor that may be used as a switching device. A MOSFET is a three terminal device that includes a source region and a drain region that are separated by a channel region, and a gate electrode that is disposed adjacent the channel region. A MOSFET may be turned on or off by applying a gate bias voltage to the gate electrode. When a MOSFET is turned on (i.e., it is in its “on-state”), current is conducted through the channel region of the MOSFET between the source region and the drain region. When the bias voltage is removed from the gate electrode (or reduced below a threshold level), the current ceases to conduct through the channel region. By way of example, an n-type MOSFET has n-type source and drain regions and a p-type channel. An n-type MOSFET thus has an “n-p-n” design. An n-type MOSFET turns on when a gate bias voltage is applied to the gate electrode that is sufficient to create a conductive n-type inversion layer in the p-type channel region that electrically connects the n-type source and drain regions, thereby allowing for majority carrier conduction therebetween.
The gate electrode of a power MOSFET is typically separated from the channel region by a thin gate insulating pattern, such as a silicon oxide pattern. Because the gate electrode of the MOSFET is insulated from the channel region by the gate insulating pattern, minimal gate current is required to maintain the MOSFET in its on-state or to switch the MOSFET between its on-state and its off-state. The gate current is kept small during switching because the gate forms a capacitor with the channel region. Thus, only minimal charging and discharging current is required during switching, allowing for less complex gate drive circuitry.
The bipolar junction transistor (“BJT”) is another well known type of semiconductor transistor that is also routinely used as a switching device. A BJT includes two p-n junctions that are formed in close proximity to each other in the semiconductor material. In operation, charge carriers enter a first region of the semiconductor material (the emitter) that is adjacent one of the p-n junctions. Most of the charge carriers exit the device from a second region of the semiconductor material (the collector) that is adjacent the other p-n junction. The collector and emitter are formed in regions of the semiconductor material that have the same conductivity type. A third, relatively thin region of the semiconductor material, known as the base, is positioned between the collector and the emitter and has a conductivity type that is opposite the conductivity type of the collector and the emitter. Thus, the two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter. By flowing a small current through the base of a BJT, a proportionally larger current passes from the emitter to the collector.
BJTs are current controlled devices in that a BJT is turned “on” (i.e., it is biased so that current flows between the emitter and the collector) by flowing a current through the base of the transistor. For example, in an n-p-n BJT (i.e., a BJT that has n-type collector and emitter regions and a p-type base region), the transistor is typically turned on by applying a positive voltage to the base to forward bias the base-emitter p-n junction. When the device is biased in this manner, the hole current that flows into the base of the transistor is injected into the emitter. The holes are referred to as “majority carriers” because the base is a p-type region, and holes are the “normal” charge carriers in such a region. In response to the hole current into the emitter, electrons are injected from the emitter into the base, where they diffuse toward the collector. These electrons are referred to as “minority carriers” because electrons are not the normal charge carrier in the p-type base region. The device is referred to as a “bipolar” device because the emitter-collector current includes both electron and hole current.
A BJT may require a relatively large base current to maintain the device in its on-state. As such, relatively complex external drive circuits may be required to supply the relatively large base currents that can be required by high power BJTs. Moreover, the switching speeds of BJTs may be significantly slower than the switching speeds of power MOSFETs due to the bipolar nature of the current conduction.
A third well known type semiconductor switching device is the Insulated Gate Bipolar Transistor (“IGBT”), which is a device that combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power BJT. An IGBT may be implemented, for example, as a Darlington pair that includes a high voltage n-channel MOSFET at the input and a BJT at the output. The base current of the BJT is supplied through the channel of the MOSFET, thereby allowing a simplified external drive circuit.
There is an increasing demand for high power semiconductor switching devices that can pass large currents in their “on” state and block large voltages (e.g., hundreds or even thousands of volts) in their reverse blocking state. In order to support high current densities and block such high voltages, power MOSFETs and IGBTs typically have a vertical structure with the source and drain on opposite sides of a thick semiconductor layer structure in order to block higher voltage levels. In very high power applications, the semiconductor switching devices are typically formed in wide band-gap semiconductor material systems (herein, the term “wide band-gap semiconductor” encompasses any semiconductor having a band-gap of at least 1.4 eV) such as, for example, silicon carbide (“SiC”), which has a number of advantageous characteristics including, for example, a high electric field breakdown strength, high thermal conductivity, high electron mobility, high melting point and high-saturated electron drift velocity. Relative to devices formed in other semiconductor materials such as, for example, silicon, electronic devices formed in silicon carbide may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities.
Vertical power MOSFET and IGBT designs may have a planar gate or a trench gate design. A common planar gate design has a gate electrode on the upper surface of the device and a channel region that is located under the gate electrode. In such devices, the current flow through the channel is in a horizontal direction (i.e., the channel defines a plane that is generally parallel to the substrate). These devices may support very high blocking voltages, but typically exhibit a higher on-state resistance as the channel is narrow and hence the resistance of the channel may be relatively high. In trench gate designs, the gate electrode is formed in a trench that extends vertically into the device adjacent the source region (in an n-type device). The gate electrode may penetrate a well region in which the source region is disposed and may terminate within the drift region. In these devices, the channel is formed in a portion of the well region between the source region and the drift region such that current flow through the channel is in the vertical direction (i.e., the channel defines a plane that is generally normal to the substrate). In trench gate designs, the channel current may flow through a much larger area, which reduces the “on-resistance” of the device and thus allows the device to support higher current densities in on-state operation. One specific type of MOSFET having a trench gate structure is the UMOSFET, which refers to a vertical MOSFET having a trench that generally resembles a “U” shape.